Catalytic Decomposition of Hydrogen Peroxide by Iodide

"Elephant's Toothpaste"

• Equipment

Large graduated cylinder (500 mL) or 2 liter soda bottle, goggles, plastic tray or sheet

 

• Reagents

30% hydrogen peroxide, dishwashing detergent, saturated solution of potassium iodide -OR- solid potassium iodide

 

• Presentation

1. Place graduated cylinder or soda bottle in a plastic tray or on a large sheet of plastic.

2. Pour ~50 mL of 30% hydrogen peroxide into the cylinder or bottle.

3. Add a squirt of dishwashing detergent; agitate slightly.

4. Add ~10 mL of potassium iodide solution OR 1/4 spoonful of solid potassium iodide. (Note: The reaction is much faster with the KI solution.) Step back quickly after adding the potassium iodide.

• Hazards

30% hydrogen peroxide is a strong oxidizing agent; contact with eyes and skin should be avoided. In case of contact, flush with water for at least 15 minutes. Get medical attention if eyes are affected. Also avoid contact of hydrogen peroxide and combustible materials. 30% hydrogen peroxide must be stored in its original container.

 

• Discussion

  Oxygen is a colorless, odorless gas at room temperature and atmospheric pressure. It is formed here by the catalytic decomposition of hydrogen peroxide by the iodide ion.

  H₂O_{2 (aq)} + I^-_(aq) OI^-_(aq) + H₂O_(l)      Step 1

   

  H₂O_{2 (aq)} + OI^-_(aq) I^-_(aq) + H₂O_(l) + O _2(g)      Step 2

   

  2 H₂O_{2 (aq)} 2 H₂O_(l) + O _2(g)      Overall

  The first step is the rate limiting step of the reaction. The oxygen that is produced causes the dishwashing detergent to foam; the foam will shoot out of the container! Note that the iodide does not appear in the overall reaction. The overall reaction is exothermic; the heat produced is enough to slightly shrink the plastic of the two liter bottle.

  For an interesting twist, put a small amount of food coloring (~5-10 drops) in a strip along the wall of the soda bottle or graduated cylinder (more dramatic if done with graduated cylinder. . The resulting foam will have a stronger resemblance to toothpaste.

• References

1. L. R. Summerlin and J. L. Ealy, Jr. Chemical Demonstrations, A Sourcebook for Teachers, Washington D. C., 1985, p.71.

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